Method for producing linear or branched fatty acid esters by means of heterogeneously catalysed reactive rectification with an upstream reactor

ABSTRACT

The disclosed invention relates to a countercurrent process for the continuous esterification of C 1-22  (fatty) acids with C 1-10  monoalkanols, C 2-5  di- or trialkanols or mixtures thereof in the liquid phase in the presence of heterogeneous catalysts in a preliminary reactor ( 1 ) and in a reaction column ( 3 ), characterized in that the reaction column ( 3 ) is preceded by the preliminary reactor ( 1 ) and a separation unit ( 2 ) for the purpose of reducing the viscosity of the reaction mixture and removing the water of reaction from the system via a separation unit ( 2 ) to displace the reaction equilibrium before the reaction column ( 3 ). A further aspect of the invention concerns feeding nitrogen in at the lowermost plate of the reaction column ( 3 ) in order to increase the vapour load in the lower part of the reaction column.

FIELD OF THE INVENTION

This invention relates to a countercurrent process for the continuousesterification of C₁₋₂₂ (fatty) acids with C₁₋₁₀ monoalkanols, C₂₋₅ di-or trialkanols or mixtures thereof in the liquid phase in the presenceof heterogeneous catalysts in a preliminary reactor (1) and in areaction column (3), characterized in that the reaction column (3) ispreceded by the preliminary reactor (1) and a separation unit (2) forthe purpose of reducing the viscosity of the reaction mixture and thewater of reaction is removed from the system via a separation unit (2)to displace the reaction equilibrium before the reaction column (3).

PRIOR ART

Processes for the continuous esterification of fatty acids are known,cf. H. Stage, Chemiker-Ztg./Chem. Apparatur 87, No. 18, 661-666 (1963).This publication describes the esterification of fatty acids withmethanol and n-butanol in multiple-section reaction columns with vaporbaffle plates operated at normal pressure. However, the continuousesterification of fatty acids on an industrial scale is normally carriedout in several stages in plate columns under pressures of 5 to 30 bar(cf. DE 2503195 C).

EP 0 334 154 B1 (Henkel) describes a countercurrent process for thecontinuous homogeneously catalyzed esterification of fatty acids withalkanols in the liquid phase in a reaction column, the catalysts andfatty acids being introduced onto the uppermost plate and the alkanolsonto the lowermost plate and being reacted at a head pressure of thereaction column of 200 to 900 hPa. This process is said to avoidunwanted secondary reactions such as, for example, the dehydration ofbranched monoalkanols. In addition, in this homogeneously catalyzedprocess, the homogeneous catalyst has to be removed at the end of thereaction. This results in fairly high catalyst losses and also losses ofproduct.

WO 90/11114 (Henkel) describes a continuous process for conducting aheterogeneously catalyzed reaction.

In discontinuous heterogeneously catalyzed processes, the solid catalystis generally size-reduced by stirrers during its direct introductioninto the reactor and has to be filtered off after the reaction. Now, WO90/11114 describes a process in which the educts are premixed in areactor and subsequently introduced by pumps into an external catalystcontainer. The reaction mixture then flows through a thin-layer orfalling-film evaporator. The progress of the reaction is determined bycontinuous monitoring, for example of the acid value. A disadvantage ofthis process is that it is a discontinuous process. According to E.Fitzer, Technische Chemie, 4th Edition (1995), disadvantages ofdiscontinuous processes are the dead times occurring during filling,emptying, heating and cooling, relatively high energy costs andrelatively high labor costs.

EP 0 474 996 Al (Hüls) describes a process for the production of estersfrom alcohols and acids by liquid-phase equilibrium reactions on ionexchangers. The reaction is carried out in a preliminary reactor and arectification column with additional external reactors. On account ofthe severe thermal stressing of the ion exchanger in a reaction-typedistillation column and the difficulties involved in separation bydistillation in the presence of ion exchangers, the esterification andrectification have to be carried out in separate spaces. However, thesedisadvantages are reliably overcome by the process according to theinvention.

The problem addressed by the present invention was to provide aneconomic process for the esterification of (fatty) acids with alkanolswhich would be a continuous, heterogeneously catalyzed process free fromthe disadvantages mentioned above. In addition, the color and odor ofthe products would be improved by less exposure to heat duringproduction.

More particularly, the problem addressed by the invention was to providean improved process for the esterification of alcohols and acids in thepresence of ion exchangers with little resistance to heat where therewould be no need to separate the ion exchanger from the rectificationcolumn. In the process according to the invention, the catalyst ispresent both in the preliminary reactor and in the rectification column.Spatial separation is not necessary. The problems stated above have beensolved by the process according to the invention.

DESCRIPTION OF THE INVENTION

In the process according to the invention, the reaction mixture of(fatty) acids and alkanols is passed through a fixed-bed reactor(preliminary reactor).

A partial reaction to the corresponding esters takes place therein.After removal of the water formed during the reaction, for example by aseparation unit, this already partly reacted, far less viscous reactionmixture is introduced into the reaction column charged with catalyst.Accordingly, the viscosity of the educt stream can be clearly reduced bythe use of a preliminary reactor, so that uniform flow through the fixedbed in the column is achieved. The early removal of the water ofreaction before the further esterification reaction in the reactioncolumn displaces the reaction equilibrium in favor of the ester formed.

Improved product properties and increased yields are obtained. Thus,conversions of up to 99.9%, based on the relatively low-volatilitycomponent used, can be achieved in the described process. In addition,secondary reactions are suppressed by the process according to theinvention. In addition to the advantage of simple regeneration of thecatalyst in the preliminary reactor, the preliminary reactor also actsas a scavenger by trapping potential catalyst poisons, so that theuseful life of the catalyst in the reaction column is considerablylengthened and production costs are reduced.

The present invention relates to a countercurrent process for thecontinuous esterification of C₁₋₂₂ (fatty) acids with C₁₋₁₀monoalkanols, C₂₋₅ di- or trialkanols or mixtures thereof in the liquidphase in the presence of heterogeneous catalysts in a preliminaryreactor (1) and in a reaction column (3), characterized in that thereaction column (3) is preceded by the preliminary reactor (1) and aseparation unit (2) for the purpose of reducing the viscosity of thereaction mixture and the water of reaction is removed from the systemvia a separation unit (2) to displace the reaction equilibrium beforethe reaction column (3).

The process according to the invention is described in more detail inthe following with reference to the accompanying drawing (FIG. 1) whichillustrates a preferred installation. However, this is not intended tolimit the invention in any way.

The central element of such an installation are the preliminary reactor(1), the water separator (separation unit) (2) and the reaction column(3) with a plurality of bubble plates surmounted by a rectifying section(4). Both the preliminary reactor and the rectification column arefilled with catalyst. Acids and alkanols are passed through thecatalyst-filled preliminary reactor. A partial reaction to the estertakes place therein. The pre-esterified product becomes liquid and isalready reduced in viscosity and is passed through a separation unit (2)located between the preliminary reactor and the reaction column. Thewater of reaction formed is removed from the partly reacted reactionmixture. The equilibrium of the esterification reaction is thusdisplaced towards the product which leads to an increased yield ofesters.

Depending on the alkanol used, the water of reaction is removed by flashprocesses or separators, preferably with a falling-film evaporator orwith a phase separator. The water is differently removed according tothe alcohol used. Thus, where the short-chain alcohols methanol orethanol are used, the water is removed from the process stream togetherwith part of the alcohol by means of a flash tank. Where short-chaincarboxylic acids are used (for example acetic acid in the production oftriacetin), the water formed is removed from the stream together withpart of the acetic acid in the same way. Where other alcohols, such asbutanol or 2-ethylhexanol, are used, a phase separator may be used toremove the water. In addition, suitable membrane processes may be usedto remove the water.

The partly reacted reaction mixture is then introduced into the reactioncolumn (3). The heterogeneous catalyst is directly applied to the columnplates. The column (3) is operated on the countercurrent principle. Ingeneral, the lower-boiling components, for example alkanols, areintroduced at the bottom of the column while the higher-boilingcomponents, such as the partly reacted reaction product, are introducedat the uppermost plate of the reactive rectification column (3).However, in the esterification of glycerol with acetic acid, the acid isintroduced at the bottom of the column as the lower-boiling component.

The further esterification reaction takes place in the column. Thereaction product is removed at the bottom of the column. The crudeproduct is further worked up by distillation and optionally bydeodorization. The components removed by distillation may be returned tothe process by introduction into the preliminary reactor (1) and/or byintroduction at the bottom of the column (3).

In the rectifying section (4) surmounting the reaction column, a mixtureof low boilers (alkanol or acid) and water is distilled off. The furtherworking up of the mixture in the separation unit (5) differs accordingto the low boiler used. Thus, where the short-chain alcohols methanol orethanol are used, water is removed with the aid of an additional column.Where other alcohols, such a butanol or 2-ethylhexanol, are used, aphase separator may be used to remove the water. As another option,membrane processes may also be used, for example, to overcome azeotropicpoints in the system.

The low boiler removed is partly recycled to the column (3) or deliveredas feed to the preliminary reactor (1). The process according to theinvention is further distinguished by the fact that nitrogen is used asan additional stripping agent (entraining agent) for removing the waterof reaction. The nitrogen is introduced into the column at the lowermostplate. In addition, the use of nitrogen increases the vapor load in thelower part of the column which prevents the liquid phase from “rainingthrough”. A more favorable (fatty) acid to alcohol ratio is alsoachieved through the use of nitrogen. For a fixed, necessary vapor loadin the column, a smaller quantity of alcohol than in the conventionalmethod of operation can be used through the use of nitrogen. This leadsto economically more favorable production. At the same time, theintroduction of nitrogen through the reaction column deodorizes thereaction mixture. Products with a satisfactory odor, even withoutadditional deodorization, may be obtained in this way. This isparticularly advantageous for the use of these products in cosmetics.

Accordingly, a preferred embodiment of the process according to theinvention is characterized in that nitrogen is fed in at the lowermostplate of the reaction column (3) in order to increase the vapor load inthe lower part of the reaction column. A particularly preferredembodiment of the process according to the invention is characterized inthat nitrogen is fed in as stripping agent at the lowermost plate of thereaction column (3) for additionally removing the water of reaction.Another preferred embodiment of the process is characterized in thatnitrogen is fed in at the lowermost plate of the reaction column (3) fordeodorization.

Preliminary Reactor

In a preferred embodiment, the preliminary reactor (1) is a fixed-bedreactor. The catalyst material is retained in the fixed-bed reactor bysuitable elements, for example by wedge-wire screens.

Reaction Column and Rectifying Section

In general, reaction columns (3) suitable for the process are anytypical plate columns, such as sieve-plate columns, but especiallybubble-plate columns with high liquid levels.

Typical representatives of these columns are described in EP-B 0 033 929and in DE-A-3146142. The use of this generation of columns is ofadvantage because a lower excess of alcohol than in conventional columnsis sufficient for achieving a complete conversion. The catalyst isdirectly applied to the column plates in the reaction column (3).

Temperature and Pressure

The reaction in the fixed-bed reactor takes place at temperatures of 50to 150° C. and preferably in the range from 80 to 120° C. and underpressures of 1 to 10 bar and preferably 1 to 5 bar. The esterificationin the reactive rectification column takes place at temperatures of 50to 200° C. and preferably in the range from 80 to 150° C. and underpressures of 0.1 to 10 bar and preferably 0.1 to 5 bar.

Accordingly, a preferred embodiment of the process according to theinvention is characterized in that the esterification is carried out attemperatures in the range from 50 to 200° C. and preferably attemperatures in the range from 80 to 150° C.

Alkanols

Suitable alkanols are linear or branched monoalkanols, di- ortrialkanols or mixtures thereof. A preferred embodiment is characterizedby the use of linear or branched C₁₋₁₀, preferably C₁ monoalcohols. Suchmonoalcohols are, for example, methanol, ethanol, propanol, butanol,pentanol and hexanol and isomers thereof. In a particularly preferredembodiment, the (fatty) acids are esterified with isopropanol or2-ethylhexanol.

Suitable linear or branched C₂₋₅ di- or trialkanols are, for example,glycerol, ethanediol, propane-1,2-diol, propane-1,3-diol, butanediol andpentanediol, isomers and semiesters thereof. Accordingly, in oneadvantageous embodiment, the (fatty) acids are esterified with C₂₋₅ di-or trialkanols, preferably with C₂₋₃ di- or trialkanols, moreparticularly with glycerol.

(Fatty) Acids

Suitable starting materials for the production of the esters are (fatty)acids with a total of 1 to 22 carbon atoms. In the context of theinvention, (fatty) acids are understood to be both mono- and polybasiccarboxylic acids and aliphatic fatty acids.

Suitable carboxylic acids are formic acid, acetic acid and adipic acid,dodecanedioc acid, citric acid, isophthalic acid.

Aliphatic fatty acids are understood to be aliphatic carboxylic acidscorresponding to the following formula:R¹CO—OH   (I)in which R¹CO is an aliphatic, linear or branched acyl group containing6 to 22 carbon atoms and 0 and/or 1, 2 or 3 double bonds. Typicalexamples are caproic acid, caprylic acid, 2-ethylhexanoic acid, capricacid, lauric acid, isotridecanoic acid, myristic acid, palmitic acid,palmitoleic acid, stearic acid, isostearic acid, oleic acid, elaidicacid, petroselic acid, linoleic acid, linolenic acid, elaeostearic acid,arachic acid, gadoleic acid, behenic acid and erucic acid and thetechnical mixtures thereof obtained, for example, in the pressurehydrolysis of natural fats and oils, in the reduction of aldehydes fromRoelen's oxo synthesis or in the dimerization of unsaturated fattyacids.

Technical C₁₂₋₁₈ fatty acids such as, for example, coconut oil, palmoil, palm kernel oil or tallow fatty acid are preferred. Headfractionated fatty acids containing 6 to 12 carbon atoms, which areobtained in large quantities in the working up of fatty acid mixtures ofnatural origin, are also preferably used.

A preferred embodiment is characterized by the use of C₆₋₂₂ (fatty)acids, preferably C₈₋₁₈ (fatty) acids and more particularly C₁₀₋₁₆(fatty) acids.

Another advantageous embodiment of the process according to theinvention is characterized in that C₁₋₅ carboxylic acids are esterifiedwith C₂₋₃ di- or trialkanols; more particularly acetic acid isesterified with glycerol.

Catalyst

The catalysts used in the process according to the invention areselected from the group consisting of organic or inorganic, basic oracidic anion or cation exchangers or acidic clays, zeolites or speciallyworked up bleaching earths and catalysts based on transition metals. Theuse of highly acidic cation exchangers, catalysts based on transitionmetal oxides or organofunctional polysiloxanes is particularlypreferred.

In one particular embodiment, acidic cation exchangers are preferablyused as the catalyst. One example of such a catalyst is Amberlyst 17(XE-386), a product of Rohm & Haas.

The higher-boiling component (for example (fatty) acids) and thelower-boiling component (for example monoalkanols) are preferably usedin a molar ratio—calculated for the reaction of a carboxyl group with ahydroxyl group—of at most 1:2, preferably 1:1.5 and more particularly1:1, i.e. the lower-boiling component should be present in an at mosttwofold excess.

Where polybasic (fatty) acids or di- or trialkanols are used, the molarratio has to be multiplied and adapted accordingly taking the desiredproducts (partial and/or full esters) into consideration. Again, thelower-boiling component should be present in an at most twofold excess.

It is pointed out by way of example that, for the reaction of glycerol(higher-boiling component) with acetic acid (lower-boiling component),the two components are used in a ratio of at most 1:6 (i.e. twofoldexcess of the acetic acid), preferably 1:4.5 and more particularly 1:3.

EXAMPLES Example 1

Amberlyst 17 (XE-386), a product of Rohm & Haas, was used as thecatalyst for the reaction. The catalyst was accommodated in a heatedglass double jacket (Vmax=200 ml, D=20 mm) as a fixed-bed reactor and inthe reaction column (27 bubble plates, D=50 mm, 1 bubble cap per plate).The following esterification of acetic acid with glycerol was carriedout at 85° C. The glycerol was fed into the preliminary reactor togetherwith part of the acetic acid (AA/Gly=4:1 [mol:mol]). The volumetric flowrate was 169 g/h. After the first reaction stage, the conversionamounted to 56%. The pre-esterified product was fed in as a liquid atthe uppermost plate of the reaction column. The acetic acid wassuperheated in countercurrent and used in vapor form with a volumetricflow rate of 268 g/h. Acetic acid/water was removed at the head of thecolumn and the reaction product at the bottom of the column. The headpressure in the column was 200 mbar. The conversion, based on glycerol,amounted to 88.7% at the bottom of the column. In order to achieve thedesired conversion of >98%, the components were passed through thecolumn a second time, an almost complete conversion (99.9%), based onglycerol, being obtained. The operating conditions were similar to thosefor the first passage. The composition of the product samples after thefirst and second passages is shown in Table 1. For working up, thereaction product was after-reacted with acetic anhydride, acetic acidwas removed and triacetin was distilled. TABLE 1 Composition of theproduct samples Percentages by weight [% by wt.] Pas- Acetic sage acidGlycerol Water Monoacetin Diacetin Triacetin 1 52.0 0.1 0.6 0.3 13.733.4 2 50.6 — <0.1 — 0.7 48.7

Example 2

The procedure described in Example 1 was repeated using Amberlyst 17(XE-386), a product of Rohm & Haas, as catalyst for the reaction. Thefollowing esterification of acetic acid with glycerol was carried out at85° C. The glycerol was fed into the preliminary reactor together withpart of the acetic acid (AA/Gly=1:1 [mol:mol]). The volumetric flow ratewas 174 g/h. After the first reaction stage, the conversion amounted to23.5%. The pre-esterified product was fed in as a liquid at theuppermost plate of the reaction column. The acetic acid was used asvapor (countercurrent) with a volumetric flow rate of 262 g/h. Aceticacid/water was removed at the head of the column and the reactionproduct at the bottom of the column. The conversion, based on glycerol,amounted to 65% at the bottom of the column. For working up, thereaction product was after-reacted with acetic anhydride, acetic acidwas removed and triacetin was distilled.

1-12. (canceled)
 13. A countercurrent process for the continuousesterification of C₁₋₂₂ fatty acids with C₁₋₁₀ monoalkanols, C₂₋₅ di- ortrialkanols or mixtures thereof, said process comprising (a) partiallyreacting the fatty acids and alkanols in a preliminary reactor in theliquid phase in the presence of an heterogeneous catalyst, (b) passingthe partially reacted reaction mixture to a separation unit, (c)removing water from the partially reacted reaction mixture in theseparation unit, (d) passing the resulting de-watered, partially reactedreaction mixture to a countercurrent reaction column, (e) furtherreacting the fatty acids and alkanols in the countercurrent reactioncolumn in the liquid phase in the presence of heterogeneous catalysts,and (f) removing the crude product from the bottom of the reactioncolumn.
 14. A process according to claim 13, further comprisingincreasing the vapour load in the lower part of the reaction column byfeeding nitrogen into the reaction column at the lowermost plate.
 15. Aprocess according to claim 13, wherein the preliminary reactor is afixed-bed reactor.
 16. A process according to claim 13, wherein theesterification reaction is carried out at temperatures of 50 to 200° C.17. A process according to claim 16, wherein the esterification reactionis carried out at temperatures of 80 to 150° C.
 18. A process accordingto claim 13, wherein the fatty acids are esterified with C₁₋₁₀monoalkanols.
 19. A process according to claim 18, wherein the fattyacids are esterified with C₁₋₈ monoalkanols.
 20. A process according toclaim 18, wherein the fatty acids are esterified with isopropanol or2-ethylhexanol.
 21. A process according to claim 13, wherein the fattyacids are esterified with C₂₋₅ di- or trialkanols.
 22. A processaccording to claim 21, wherein the fatty acids are esterified with C₂₋₃di- or trialkanols.
 23. A process according to claim 21, wherein thefatty acids are esterified with glycerol.
 24. A process according toclaim 13, wherein the esterification catalysts used are selected fromthe group consisting of organic or inorganic, basic or acidic anion orcation exchangers or acidic clays, zeolites or specially worked upbleaching earths and catalysts based on transition metals.
 25. A processaccording to claim 13, wherein acidic cation exchangers are used as thecatalyst.